Zeolites are crystalline aluminosilicate compositions which are microporous and which are formed from corner sharing [AlO4/2]− and SiO4/2 tetrahedra. Numerous zeolites, both naturally occurring and synthetically prepared are used in various industrial processes. Synthetic zeolites are prepared via hydrothermal synthesis employing suitable sources of Si, Al and structure directing agents (SDAs) such as alkali metals, alkaline earth metals, amines, or organoammonium cations. The structure directing agents reside in the pores of the zeolite and are largely responsible for the particular structure that is ultimately formed. These species balance the framework charge associated with aluminum and can also serve as space fillers. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure. Zeolites can be used as catalysts for hydrocarbon conversion reactions, which can take place on outside surfaces of the zeolite as well as on internal surfaces within the pores of the zeolite.
In 1982, Wilson et al. developed aluminophosphate molecular sieves, the so-called AlPOs, which are microporous materials that have many of the same properties of zeolites, but are silica free, composed of [AlO4/2]− and [PO4/2]+ tetrahedra (See U.S. Pat. No. 4,319,440). Subsequently, charge was introduced to the neutral aluminophosphate frameworks via the substitution of SiO4/2 tetrahedra for [PO4/2]+ tetrahedra to produce the SAPO molecular sieves (See U.S. Pat. No. 4,440,871). Another way to introduce framework charge to neutral aluminophosphates is to substitute [M2+O4/2]2− tetrahedra for [AlO4/2]− tetrahedra, which yield the MeAPO molecular sieves (see U.S. Pat. No. 4,567,029). It is furthermore possible to introduce framework charge on AlPO-based molecular sieves via the introduction both of SiO4/2 and [M2+O4/2]2− tetrahedra to the framework, giving MeAPSO molecular sieves (See U.S. Pat. No. 4,973,785).
In the early 1990's, high charge density molecular sieves, similar to the MeAPOs but without the Al, were developed by Bedard (See U.S. Pat. No. 5,126,120) and Gier (See U.S. Pat. No. 5,152,972). These metal phosphates (sometimes arsenates, vanadates) were based on M2+ (M=Zn, Co), the general formula of which, in terms of the T-atoms, T2+-T5+, was approximately A+T2+T5+O4, having framework charge densities similar to Si/Al=1 zeolites and were charge balanced by alkali cations, A+, in the pores. Later attempts to prepare metallophosphates of similar compositions but with organic SDAs led to porous, but interrupted structures, i.e., the structures contained terminal P—O—H and Zn—N bonds (See J. Mater. Chem., 1992, 2(11), 1127-1134.) Attempts at Al substitution in a zincophosphate network was carried out in the presence of both alkali and organoammonium agents, specifically the most highly charged organoammonium species, tetramethylammonium, but because of the high framework charge density, only the alkali made it into the pores to balance framework charge (See U.S. Pat. No. 5,302,362). Similarly, in a high charge density zincophosphate system that yielded the zinc phosphate analog of zeolite X, the synthesis in the presence of Na+ and TMA+ yielded a product that contained considerably less TMA+ than Na+ (See Chem. Mater., 1991, 3, 27-29).
To bridge the rather large charge density gap between the MeAPOs of U.S. Pat. No. 4,567,029 and the aforementioned alkali-stabilized Me2+-phosphates of Bedard and Gier, Stucky's group developed a synthesis route using amines, often diamines in ethylene glycol solvent. They were able to make high charge density, small pore MeAPOs in which the concentrations of Co2+ and Al3+ in R(CoxAl1-x)PO4 were varied such that 0.33≤x≤0.9 in the so-called ACP series of materials, the aluminum cobalt phosphates (See Nature, 1997, 388, 735). Continuing with this synthesis methodology utilizing ethylene glycol solution matching the amines to framework charge densities for R(M2+xAl1-x)PO4, such that 0.4≤x≤0.5, (M2+=Mg2+, Mn2+, Zn2+, Co2+) the large pore materials UCSB-6, -8 and -10 were isolated (See Science, 1997, 278, 2080). Crystal dimensions isolated from that work were often on the order of hundreds of microns. Similarly, this approach also yielded MeAPO analogs of zeolite rho of the composition RM2+0.5Al0.5PO4, where R═N, N′-diisopropyl-1, 3-propanediamine, M2+=Mg2+, Co2+ and Mn2+. Cowley followed this ethylene glycol-based strategy to prepare the cobalt and zinc gallium phosphates using quinuclidine as the SDA, that have the CGS topology and a framework charge density of −0.125/T-atom (See Microporous and Mesoporous Materials 28, 1999 163-172). Similarly, Lin and Wang used 1,2 diaminocyclohexane (DACH) with the ethylene glycol approach to prepare a Zn—Ga phosphate of CGS topology with higher Zn incorporation than the Cowley work, realizing a framework charge density of −0.25/T-atom for (H2DACH)Zn2Ga2(PO4)4 (See Chemistry of Materials, 12, 2000 3617-3623). The reliance of these synthesis approaches on an ethylene glycol solvent does not lend itself well to industrial scale, from both a safety and environmental point of view. Other than this work, there has been little activity in this intermediate charge density region, where 0.2≤x≤0.9 for the [M2+x(Al,Ga)1-xPO4]x− compositions.
Pursuing aqueous chemistry, Wright et al. used highly charged triquaternary ammonium SDAs to make new MeAPO materials (See Chem. Mater., 1999, 11, 2456-2462). One of these materials, STA-5 with the BPH topology, (Mg2.1Al11.9P14O28), exhibited significant substitution of Mg2+ for Al3+, up to about 15%, but less substitution than seen in Stucky's non-aqueous ethylene glycol approach.
More recently, Lewis et al. developed aqueous solution chemistry using quaternary ammonium cations leading to high charge density SAPO, MeAPO, and MeAPSO materials, enabling greater substitution of SiO4/2 and [M2+O4/2]2− into the framework for [PO4/2]+ and [AlO4/2]−, respectively, using the ethyltrimethylammonium (ETMA+) and diethyldimethylammonium (DEDMA+) SDAs. These materials include ZnAPO-57 (U.S. Pat. No. 8,871,178), ZnAPO-59 (U.S. Pat. No. 8,871,177), ZnAPO-67 (U.S. Pat. No. 8,697,927), and MeAPSO-64 (U.S. Pat. No. 8,696,886). The relationship between the increasing product charge densities and reaction parameters, namely the ETMAOH(DEDMAOH)/H3PO4 ratios, were outlined in the literature (See Microporous and Mesoporous Materials, 189, 2014, 49-63). The incorporation of M2+ observed in these systems was such that for the formulation [M2+xAl1-xPO4]x−, x˜0.3, about 30% substitution of Al and a framework charge density of −0.15/T-atom.
Applicants have now synthesized a new family of highly charged metallophosphate framework materials, designated PST-16, with higher charge densities than the MeAPOs of U.S. Pat. No. 4,567,029 and the ZnAPO materials isolated by Lewis. Contrary to the work cited above in which high framework charge density metallophosphates are prepared from amines in ethylene glycol solution, the current materials are prepared from aqueous solution utilizing a combination quaternary ammonium and alkali cations. The PST-16 materials have the CGS topology (See Database of Zeolite Structures, www.iza-structure.org/databases) and exhibit a range of framework charge densities that are greater than those observed by Cowley and comparable to or higher than those observed by Lin and Wang. The utility of alkali in metalloaluminophosphate-based systems is uncommon and in combination with quaternary ammonium cations under the right conditions enables this system to achieve the charge densities and desired midrange compositions between the low charge density MeAPOs of U.S. Pat. No. 4,567,029 and high charge density M2+-phosphate extremes.